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Borders come from the template you have chosen. You can remove it by editing the HTML.
In the HTML find the text that looks like:
<img src="https://sites.google.com/site/...image.gif" border="0">
change it by adding the bold text:
<img src="https://sites.google.com/site/..image..gif" border="0" style="border-width: 0px; background-color: transparent;">

 

 

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